WO2017187323A1

WO2017187323A1 - Busbar system for electrolytic cells arranged side by side in series

Info

Publication number: WO2017187323A1
Application number: PCT/IB2017/052351
Authority: WO
Inventors: Vinko Potocnik; Marwan ALBASTAKI; Abdalla ALZAROONI
Original assignee: Dubai Aluminium Pjsc
Priority date: 2016-04-26
Filing date: 2017-04-25
Publication date: 2017-11-02
Also published as: GB2549731A

Abstract

A busbar arrangement for two adjacent upstream and downstream electrolytic cells, each electrolytic cell comprising a bottom cathode comprising cathode blocks, each cathode block being provided with a current collector bar and two electrical connections points, and each cell further comprising anodes connected to an anode busbar. Said arrangement further comprises a cathode busbar system (2-5) comprising two opposite longitudinal parts (2,3) extending along the long sides of the upstream cell, and two opposite transversal parts (4,5) extending along the ends of the upstream cell, and an anode busbar system (315) comprising two opposite longitudinal parts (316,317) and at least two opposite transversal parts (318a, 318d) extending between said longitudinal parts (316,317). According to the invention said arrangement further comprises anode risers (10) connecting the cathode busbar (2-5) of an upstream cell to the anode busbar (315) of a downstream cell, said anode riser comprising a first section (11, 14) and a second section (12, 13), each section being provided with respective first and second connection means on cathode busbar and with respective first and second connection means on anode busbar, said anode riser further comprising means (19) for electric insulation between said first section and said second section.

Description

BUSBAR SYSTEM FOR ELECTROLYTIC CELLS ARRANGED SIDE BY SIDE

IN SERIES

Technical field of the invention

The invention relates to the field of fused salt electrolysis, and more precisely to an electrolytic cell suitable for the Hall-Heroult process for making aluminium by fused salt electrolysis. In particular, the invention relates to a particular arrangement of the cathode busbar system in an electrolysis plant in which electrolytic cells are arranged side-by-side, capable of balancing the electrical current distribution in cathode collector bars and busbars with reduced busbar mass and reduced pot-to-pot distance. This busbar system uses split anode risers as an integral part of the balancing circuit.

Prior art

The Hall-Heroult process is the only continuous industrial process for producing metallic aluminium form aluminium oxide. Aluminium oxide (Al₂0₃) is dissolved in molten cryolite (Na₃AIF₆), and the resulting mixture (typically at a temperature comprised between 940 °C and 970 °C) acts as a liquid electrolyte in an electrolytic cell. An electrolytic cell (also called "pot") used for the Hall-Heroult process typically comprises a steel shell (so-called pot shell), a lining (comprising refractory bricks protecting said steel shell against heat, and cathode blocks usually made from graphite, anthracite or a mixture of both), and a plurality of anodes (usually made from carbon) that plunge into the liquid electrolyte. Anodes and cathodes are connected to external busbars. An electrical current is passed through the cell (typically at a voltage between 3.5 V and 5 V) which electrochemically reduces the aluminium oxide, split by the electrolyte into aluminium and oxygen ions, into aluminium at the cathode and oxygen at the anode; said oxygen reacting with the carbon of the anode to form carbon dioxide. The resulting metallic aluminium is not miscible with the liquid electrolyte, has a higher density than the liquid electrolyte and will thus accumulate as a liquid metal pad on the cathode surface below the electrolyte from where it needs to be removed from time to time, usually by suction into a crucible.

The electrical energy is a major operational cost in the Hall-Heroult process. Capital cost is an important issue, too. Ever since the invention of the process at the end of the 19^th century much effort has been undertaken to improve the energy efficiency (expressed in kW/h per kg or ton of aluminium), and there has also been a trend to increase the size of the pots and the current at which they are operated in order to increase the plant productivity and bring down the capital cost per unit mass of aluminium produced in the plant. Industrial electrolytic cells used for the Hall-Heroult process are generally rectangular in shape and connected electrically in series, the ends of the series being connected to the positive and negative poles of an electrical rectification and control substation. The general outline of these cells is known to a person skilled in the art and will not be repeated here in detail. They have a length usually comprised between 8 and 25 meters and a width usually comprised between 3 and 5 meters. The cells (also called "pots") are always operated in series of several tens (up to several hundreds) of pots (such a series being also called a "potline"); within each series DC currents flow from one cell to the neighbouring cell. For protection the cells are arranged in a building, with the cells arranged in rows either side-by-side, that is to say that the long side of each cell is perpendicular to the axis of the series, or end-to-end, that is to say that the long side of each cell is parallel to the axis of the series. It is customary to designate the sides for side- by-side cells (or ends for end-to end cells) of the cells by the terms "upstream" and "downstream" with reference to the orientation of current flow in the series. The current enters the upstream and exits downstream of the cell. The electrical currents in most modern electrolytic cells using the Hall-Heroult process exceed 200 kA and can reach 400 kA, 450 kA or even more; in these potlines the pots are arranged side by side. Most newly installed pots operate at a current comprised between about 350 kA and 600 kA, and more often in the order of 400 kA to 500 kA.

These enormous electrical DC currents flow through various conductors, such as electrolyte, liquid metal, anodes, cathode, connecting conductors, where they generate heat with ohmic voltage drops and where they generate significant magnetic fields. As mentioned above, electrolysis according to the Hall-Heroult process is a continuous process driven by the flow of electric current across the electrolyte, whereby said electric current reduces the aluminium atoms that are bounded in the alumina added to the molten electrolyte. Four equilibria define the optimum cell operation window, leading to the high current efficiency: electrical equilibrium, magnetic equilibrium, thermal equilibrium and chemical equilibrium; the first two of these equilibria are determined by the cell design, the other two can be acted upon in the process of cell operation.

Conditions of electrical equilibrium of the cell are attained when the current distribution is as uniform as possible throughout the electrolyte; the thickness of electrolyte between the anode and the liquid metal pad which acts as cathode (inter-electrode spacing) in a typical Hall-Heroult cell is of the order of about two to five centimeters. The main operating parameter by which the operator can act on this equilibrium is the inter-electrode spacing; which determines the cell voltage to a large extent. The main permanent perturbation factor of the electrical equilibrium is the current path in the liquid metal, as will be explained below; this factor is determined by the cell design and cell operation. Discontinuous events such as anode change and so-called anode effects also perturb the electrical equilibrium. The magnetic equilibrium and the thermal equilibrium of the cell are both determined to a large extent by the cell design. The magnetic equilibrium is determined to a large extent by the busbar structure. Perturbation factors are mainly related to electrical currents arising from conductors outside of the cell. The thermal equilibrium is determined by the choice and thickness of materials and components, and by the lining; perturbation factors are mainly related to specific discontinuous operations (anode change, metal tapping, adding of electrolyte) or to so-called anode effects (this term and the phenomenon that it designates are known to a person skilled in the art and need not to be explained here).

The chemical equilibrium is determined by the chemical composition of the electrolytic bath; alumina and aluminium fluoride additions are the principal operational parameters for control of chemical equilibrium.

The present invention is related to the electrical and magnetic equilibrium of an electrolytic Hall-Heroult cell. Such cells are of rectangular shape, and as such are symmetric by construction. Asymmetry arises from asymmetry of the electric current flow in the cell. Electrical current enters the cell through anodes which cover a large part of the surface of the cell, crosses the electrolyte and the liquid metal pad, and is collected by the cathode carbon blocks which form the whole surface of the cell bottom. The cathode is made of a carbonaceous material and contains steel (or other high temperature resistant metallic) collector bars which enable an electrical contact to be established with the cathode busbar. However, the electrical conductivity of both the cathode and the steel cathode bar is much lower than that of the liquid metal pad. As a consequence the current lines in the liquid metal are not vertical but have horizontal components, interacting with the vertical magnetic field and leading to magnetohydrodynamic (MHD) perturbations. Furthermore, at the downstream side of the cell, the cathode busbar is linked to a small number of connecting elements called anode risers, through which the current is fed into the anode beam of the downstream cell.

As a consequence, the magnetic field in the cell locally has a spatial distribution which, combined with electrical currents in the cell, creates Laplace forces; these induce, movement of liquid conductors (electrolyte and metal) and deform the bath metal interface hydrostatically. The Laplace forces may also induce metal-bath interface oscillations. The resulting unevenness of the metal surface leads to a local variation in anode-to-cathode distance across the length of the pot, which is represented by small fluctuations of the overall cell voltage signal; this may even lead to a short-circuit between the anode and the cathode. These oscillations of the metal-bath interface are called magnetohydrodynamic (MHD) instabilities which are detrimental to the performance of the process; they require the an increase of anode-to-cathode distance and this counter measure increases the electrical resistance of the cell, leading to ohmic losses and an increase in energy consumption. The MHD instabilities are specifically the result of the vertical magnetic field component which tends to increase with the size of the pots and with the cell current. Certain perturbative events (adding alumina, anode change, metal tapping, anode effects) may increase these instabilities and metal and bath velocities, The perturbative effects of an event will be higher if the vertical component of the magnetic field, in particular in the upstream corners of the cell, is high. It is therefore desirable, in order to reduce these magnetohydrodynamic instabilities, to decrease as far as possible the vertical component of the magnetic field (B_z) in the liquid metal; a root-mean square average value of about one millitesla is a usual maximum target. Moreover, the horizontal components B_x and B_y (x being the longitudinal axis of the cell) should be anti-symmetrical with respect to the longitudinal and transverse axis, respectively. A required property of B_z is also the anti-symmetry with respect to the cell centre, i.e. equal and opposite values in each corner of the cell.

Another perturbation factor of a cell operating under conditions of magnetic equilibrium is the effect of neighboring cells, as cells are usually arranged side-by-side in series of up to several hundred cells and divided in at least two potrooms. This perturbation leads to local variations of the vertical component of the magnetic field B_z, (z being the coordinate running upwards from the bottom to the top of the cell) in the liquid metal pad which destroy the anti-symmetry of B_z with respect to cell centre, required for good MHD stability of the cell. More precisely, the value of the vertical magnetic field should be zero in the geometrical center of the liquid metal pad, but the contribution from the adjacent rows gives a bias that can be greater than one millitesla, depending on potline current and distance to the adjacent rows of cells. The magnetic effect of neighboring cells can be decreased by an appropriate design of the potline, and prior art offers a wide range of such designs. As an example, US 4,169,034 discloses the use of two compensation loops which produce an additional compensating magnetic field substantially equal to that created by the adjacent rows. US 4,683,047 achieves the same goal with asymmetric busbars below the cell. It should be borne in mind that the simple upscaling of a cell is usually not possible without specifically adapting the whole structure of electrical distribution system, as MHD effects tend to increase with increasing current. The main starting point for such a design is the number and position of anode risers, and the design of the cathode busbar system. The design of the cathode busbar system and of anode risers has two aims.

The first aim is to generate nearly equal electrical currents in all upstream and downstream cathode collector bars. This aim is achieved by providing equal electrical resistance between upstream and downstream cathode busbars from liquid metal pad to the common electrical point on the downstream side of the cell. A typical plot of collector bar currents is given in Figure 10. Since the distance between upstream side of the cell to the common electrical point is much greater than from the downstream side to the common electrical point, the busbar cross-sections of the upstream busbars must be much greater than of the downstream busbars. In the prior art, described in US 6 551 473, the downstream busbars are made longer by zig-zag arrangement before they reach the common point. This requires more space between the cells and increases the busbar mass: both aspects tend to increase the investment cost of a potline.

The second aim is to obtain compensated vertical magnetic field B_z in the cells, and especially in the upstream corners where B_z is usually the highest. A typical plot of B_z over the length of the cell is shown on Figure 7.

Several patents have been published which present a design in which the magnetic fields created by the various parts of the cell and the connecting conductors compensate one another, thus decreasing magnetohydrodynamic instabilities in the cell. The targeted result is a cell having a magnetic field in the cell which is symmetric or anti-symmetric with respect to the cell axes or the cell centre as explained above.

The present invention focuses on providing electrical equilibrium in a cell with reduced cell-to-cell distance and reduced busbar mass while keeping the desired magnetic equilibrium of the cell.

Object of the invention

According to the invention, the problem of electrical equilibrium while reducing the cell to cell distance in side-by-side cell arrangements has been solved in a surprising manner by a modification of both the cathode busbar system and of anode risers. Cathode busbar systems in side-by-side cells can be symmetric or asymmetric with respect to a median transverse plane. The inventors have found that the vertical magnetic field in the upstream corners of a cell can be decreased in a significant way by using straight cathode busbar running along the end of the cell and by using a novel anode riser design. The present invention applies to electrolytic cells of substantially rectangular shape, suitable for the Hall-Heroult electrolysis process, arranged side-by-side, in which the cathode busbar is connected to a plurality of risers through which the current is fed into the anode beam of the downstream cell. Said plurality of risers is arranged lengthwise, that is to say for a given cell said risers are arranged between the downstream rim of the cell and the upstream rim of the adjacent downstream cell. According to the invention, each riser is split into two on the whole length from the cathode busbars to the common point on the anode beam. The common point can be on the upstream anode beam or more advantageously, the upstream risers are connected to the upstream anode beam and the downstream risers connected to the downstream anode beam of the adjacent downstream cell. At the base of the risers, the downstream riser is connected to the downstream cathode busbar of the cell and the upstream riser to the upstream cathode busbar of the cell. This arrangement provides a much longer straight path from the downstream side to the anode beam than in the prior art. The need for zig-zag busbars on the downstream side of the cell is therefore eliminated and this is one of the main advantages of the present invention.

A first object of the invention is therefore an anode riser for an electrolytic cell, in particular for a cell suitable for the Hall-Heroult electrolysis process, said riser being intended to electrically connect a cathode busbar of an upstream cell and an anode busbar of a downstream cell,

said anode riser comprising a first section and a second section, each section being provided with respective first and second connection means on cathode busbar and with respective first and second connection means on anode busbar,

said anode riser further comprising means for electric insulation between said first section and said second section.

Said means for electric insulation comprise one or more insulating materials filling at least part of an intercalary space defined by facing so called split walls of first and second sections. Said insulating material can be a plate made in a suitable polymer material, or air. Said intercalary space advantageously has a substantially constant thickness over at least part of the length of said riser, at least in the vicinity of first connection means. Said thickness is typically comprised between 2 mm and 20 mm. In one embodiment said riser comprises a rigid mast, intended to be connected on cathode busbar, as well as a flexible top, extending from said mast and being intended to be connected on anode busbar, said mast comprising a first mast section and a second mast section, said top comprising a first top section and a second top section. Said riser comprises first fixing means between first mast section and first top section, as well as second fixing means between second mast section and second top section. Each fixing means can be a removable fixing means. Each removable fixing means can comprise a fixing plate, said plate being permanently fixed, in particular by welding, to one amongst mast section and top section, and being removably fixed, in particular by bolting, to the other amongst mast section and top section. Said first top section and second top section can have substantially the same length. In one embodiment one amongst said first top section and second top section is substantially longer than the other, so that said first top section and second top section may directly contact different portions of anode bus bar.

Another objet of the present invention is a busbar arrangement for two adjacent upstream and downstream electrolytic cells of substantially rectangular shape, in particular for cells suitable for the Hall-Heroult electrolysis process,

each electrolytic cell comprising a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks, each cathode block being provided with at least one current collector bar and two electrical connections points,

a lateral lining defining together with the cathode a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process,

said cathode and lateral lining being contained in an outer metallic shell,

and each electrolytic cell further comprising a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one anode and at least one metallic anode rod connected to an anode busbar (so-called anode beam),

said arrangement comprising a cathode busbar system as well as an anode busbar system,

said cathode busbar system comprising two opposite longitudinal parts each intended to extend along the long sides of the upstream cell, and two opposite transversal parts intended to extend along the ends of the upstream cell, said longitudinal and transversal parts forming a so-called "ring busbar" system, said anode busbar system comprising two opposite longitudinal parts and at least two opposite transversal parts extending between said longitudinal parts,

said arrangement further comprising at least one anode riser according to the first object of the invention and any of its embodiments and variants.

One of said longitudinal parts of said cathode busbar system can comprise first and second electrically isolated sections, the first section of said riser being connected to first section of said longitudinal part, whereas second section of said riser is connected to second section of said longitudinal part.

In one embodiment the first section and the second section of said riser are connected to a first longitudinal part of the anode bus bar of a neighbouring downstream cell.

In another embodiment the first section of said riser is connected to a first longitudinal part of the anode bus bar, whereas second the section of said riser is connected to a second longitudinal part of the anode bus bar of a neighbouring downstream cell.

In another embodiment said two parallel longitudinal parts of the cathode busbar system are the downstream longitudinal part, electrically connected to the downstream anode risers, and the upstream longitudinal part connected to the said upstream anode risers via the busbars at the ends of the cell.

In preferred embodiments, the parallel transversal parts of the cathode busbar are symmetric with respect to said median longitudinal plane and essentially straight between upstream and downstream of the cell.

As cathode blocks are symmetric and have collector bar ends coming out on each side, in side-by-side arrangements of electrolytic cells approximately half of the current collected by the collector bars of the cathode blocks will flow directly to the downstream longitudinal part of the cathode busbar system, while the other approximately half flows to the upstream longitudinal part. It is therefore necessary to carry the cathode current collected at the upstream side of the cathode busbar system (that is to say by the upstream longitudinal parts) to the downstream segment of the cathode busbar system which is connected to one of the two parts of the split risers. In one embodiment of the invention this is achieved by the transversal parts of the cathode busbar. However, such a busbar circuit needs to be equilibrated because the path of the current collected by the upstream longitudinal parts is longer than the path of the current collected by the downstream longitudinal parts and this in spite of shifting the common point between upstream and downstream busbars to the anode beam. Furthermore, it is desirable that each anode riser collects a predefined current; if said plurality of risers comprises end risers and central risers, the end risers may collect a different current than the central risers or equal current to the one in centre risers, also, upstream risers may collect a different current than the downstream risers. This can be achieved by different cross-sections of the risers.

For these reasons the cathode busbar system may comprise additional electrical balancing circuits. Said electrical balancing circuits and the components thereof are not a part of the ring busbar as defined herein. In one embodiment which can be combined with any of the previous ones, said cathode busbar system further comprises two or more conductive arms that extend between said longitudinal parts of said ring busbar, underneath said shell. These conductive arms extending underneath the ring busbar system connect the upstream longitudinal part of the ring busbar to the downstream longitudinal segment connected to the upstream risers, thereby creating an additional path for the cathode current collected upstream. They are not part of the ring busbar system as such; they act as an upstream electrical balancing circuit, achieving preferential feeding of the cathode current collected by the upstream longitudinal parts of said ring busbar to the upstream risers. Said conductive arms can be symmetric or asymmetric with respect to said median longitudinal planes, and/or they can be symmetric or asymmetric with respect to said median transversal plane. In an advantageous embodiment, said arms are asymmetric with respect to said median longitudinal plane. As per object of this invention, said balancing circuits do not require conductors arranged in vicinity of and parallel (referring to zig-zag); the downstream risers are attached directly to the downstream longitudinal part of said ring busbar. The electrical balance is achieved by proper selection of the cross-section of the downstream risers.

Another object of the invention is an electrolytic cell of substantially rectangular shape suitable for the Hall-Heroult electrolysis process, comprising

a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks, each cathode block being provided with at least one current collector bar and two electrical connections points,

a lateral lining defining together with the cathode a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process, said cathode and lateral lining being and lining being contained in an outer metallic shell, a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one anode and at least one metallic anode rod connected to an anode beam,

said electrolytic cell being characterized in that it comprises a riser according to the invention and any of its embodiments and variants.

Yet another embodiment is an aluminium electrolysis plant comprising at least one line (L1 , L2) of electrolysis cells (C1 , Cn) of substantially rectangular shape, said cells being arranged side by side,

each cell comprising

a lateral lining defining together with the cathode a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process, said cathode and lateral lining being and lining being contained in an outer metallic shell,

a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one anode and at least one metallic anode rod connected to an anode beam,

said plant further comprising means for electrically connecting said cells in series and for connecting the cathode busbar of a cell to the anode beam of a downstream cell, characterized in that more than 80 % of the electrolysis cells in at least one line (L1 , L2), and preferably each electrolysis cell in said line, is an electrolysis cell according to the present invention.

Yet another objet is an aluminium electrolysis plant comprising at least one line (L1 , L2) of electrolysis cells (C1 , Cn) of substantially rectangular shape, said cells being arranged side by side,

each cell comprising

a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one anode and at least one metallic anode rod connected to an anode beam, said plant further comprising means for electrically connecting said cells in series and for connecting the cathode busbar of a cell to the anode beam of a downstream cell, characterized in that more than 80 % of the couple of adjacent electrolysis cells in at least one line (L1 , L2), and preferably each couple of electrolysis cells in said line are electrically connected by a busbar arrangement according to the invention and any of its embodiments and variants.

A final object of the invention is a process for making aluminium by the Hall-Heroult electrolysis process using electrolytic cells of substantially rectangular shape, characterized in that said process is carried out in an aluminium electrolysis plant according to the invention.

- a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks, each cathode block being provided with at least one current collector bar and two electrical connections points,

said cathode and lateral lining being and lining being contained in an outer metallic shell,

said electrolytic cell being characterized in that it comprises a cathode busbar system according to any of the embodiments and variants of the present invention.

Another object of the invention is an aluminium electrolysis plant comprising at least one line of electrolysis cells of substantially rectangular shape, said cells being arranged side by side, and said plant further comprising means for electrically connecting said cells in series and for connecting the cathode busbar of a cell to the anode beam of a downstream cell,

characterized in that more than 80 % of the electrolysis cells in at least one of said line, and preferably each electrolysis cell of said line, is an electrolysis cell according to the present invention. A last object of the invention is a method for making aluminium by the Hall-Heroult electrolysis process using electrolytic cells of substantially rectangular shape, characterized in that said method is carried out in an aluminium electrolysis plant according to the invention.

Figures

Figures 2 to 7 represent various embodiments of the present invention. Figures 1 ,8 and 9 illustrate prior art.

Figure 1 shows schematically the global arrangement of a plant according to the invention.

Figure 2 is a perspective view, showing a cathode busbar according to a first embodiment of the invention, which belongs to the smelter of the figure 1.

Figure 3 is the same view as figure 2 (without numerical reference signs), showing the current flow represented by arrows, as well as certain geometrical reference axis and planes referred to throughout the description. Arrows represented by a continuous black line represent the downstream current flow path, and dotted arrows represent the upstream current flow path.

Figures 4 and 5 refer to the detail marked on figure 3 from a different perspective. Figure 4 shows the current flow under normal cell operation. Figure 5 shows the current flow when the anode riser leading to the downstream cell has been dismantled and the switch plate has been added to the circuit.

Figures 6 and 7 schematically represent a variant of the downstream anode beam and anode risers according to the invention: figure 6 shows the reference signs used throughout the description, while figure 7 shows the current flow represented by arrows.

Figure 8 is a schematic cross section along a transversal plane across a Hall-Heroult electrolytic cell. The arrows represent the current flow across the cell.

Figure 9 is a typical plot of the vertical magnetic field (B_z) depending on the distance from the centre point of a typical 420 kA electrolysis cell. The three curves correspond to different lines parallel to the length of the cell: curve (a) corresponds to the downstream region, curve (b) to the upstream region, curve (c) to the centre.

Figure 10 gives a typical plot of collector bar currents when the upstream and downstream busbars are well balanced.

The following reference numbers and letters are used on the figures:

Detailed description

The present invention is directed to the global arrangement of a plant, or aluminium smelter, used in the Hall-Heroult process. As schematically shown on Figure 1 , the aluminium smelter of the invention comprises a plurality of electrolytic cells C1 , C2, ... , Cn-1 , Cn, arranged the one behind the other (and side by side) along two parallel lines L1 and L2, each of which comprises n/2, i.e. m cells. These cells are electrically connected in series by means of conductors, which are not shown on Figure 1. The number of cells in a series is typically comprised between 50 and over 400, but this figure is not substantial for the present invention. The electrolysis current therefore passes from one cell to the next, along arrow DC. The cells are arranged transversally in reference of main direction D1 or D2 (axis of the row) of the line L1 or L2 they constitute. In other words the main dimension, or length, of each cell is substantially orthogonal to the main direction of a respective line, i.e. the circulation direction of current. Figure 1 depicts a typical "clockwise" current orientation.

The Hall-Heroult process as such, the way to operate the latter, as well as the cell arrangement are known to a person skilled in the art and will not be described here in more detail. In the present description, the terms "upper" and "lower" refer to mechanical elements in use, with respect to a horizontal ground surface. Moreover, unless otherwise specifically mentioned, "conductive" means "electrically conductive". The general structure of a Hall-Heroult electrolysis pot is known per se and will not be explained here. It is sufficient to explain, in particular in relation with Figure 8, that the current is fed into the anode busbar (called anode beam, not shown on the figures), flows from the anode beam to the anode rod 304 and to the anode 301 in contact with the liquid electrolyte 302 where the electrolytic reaction takes place, crosses the liquid metal pad 303 resulting from the process and eventually will be collected at the cathode block 305. As cathode blocks are symmetric and have collector bar ends 306,307 coming out on each side, in side by side arrangements of electrolytic cells approximately one half of the current collected by the collector bars of the cathode blocks will flow directly to the downstream longitudinal part 2 of the cathode busbar system, while the other approximately one half flows to the upstream longitudinal part 3. As will be explained later, means are provided to carry the cathode current collected at the upstream part 3 of the cathode busbar system back to the downstream longitudinal part 2 of the cathode busbar system. The present invention is more particularly directed to the cathode busbars of the potline, each of which surrounds a respective cell, and to the anode risers (schematically shown on Figure 3). Hereafter, the arrangement of two embodiments of the busbar associated with cell C2 will be described, in relation with figures 2 and following. Preferably, the arrangement of a majority of the other busbars and, most preferably, of all the busbars of the plant, is similar.

Turning now to Figure 2, cathode busbar as a whole is given the general reference 1. It rests on appropriate structural elements (not shown on the figures), such as columns, in a way known as such; in a known manner, said columns rest on insulating plots on a horizontal support (usually concrete) in order to electrically insulate them from the ground. Thus, this busbar system 1 is located on about the same horizontal level as the molten aluminium metal contained within the cell. The cell is designated as C2 on Figure 1. Busbar system 1 comprises different mechanical elements, which will be described hereafter more in detail. It first includes a ring (called here "ring busbar") which is generally formed by two longitudinal parts 2 and 3, parallel to axis X-X, as well as two transversal parts 4 and 5. The ring busbar is not a completely closed ring, since the part 3 of the busbar is at its ends separated from transversal parts 4 and 5 by an air gap in order to separate electrically the upstream busbars from the downstream busbars. This ring busbar defines a main plane PR, which extends horizontally. Moreover, two branches 6 and 7, which are parallel to transversal parts 4 and 5, extend between opposite longitudinal parts 2 and 3. All the elements which form busbar system 1 are made of aluminium.

The whole ring busbar 2 - 5 has a rectangular shape, the length LR of which is slightly superior to that of cell C2, whereas the width WR of which is slightly superior to that of cell C2. By way of example, length LR is between about 14,000 mm and about 25,000 mm, whereas width WR is between about 5.000 mm and about 9,000 mm. Axis X-X defines a median longitudinal direction of the cell and of the whole ring busbar 2 - 5, whereas axis Y-Y defines a median transversal, or lateral direction of the cell and of the whole ring busbar 2 - 5. As explained hereabove, transversal axis Y-Y of the ring busbar 2 - 5 corresponds to the main longitudinal direction D1 of the line L1 which includes cell C2. Moreover, PX defines a median longitudinal plane of the cell and of the whole ring busbar 2 - 5, said plane being orthogonal to main plane PR and including axis X-X. PY defines a median transversal plane of the cell and of the whole ring busbar 2 - 5, said plane being orthogonal to main plane PR and including axis Y-Y. As explained more in detail hereafter, in the embodiment of figures 2 to 6, the ring busbar is asymmetric with respect to plane PX; this is an essential feature of the present invention. The axes X-X and Y-Y, the planes PR, PY and PX, and the width and length parameters WR and LR are shown on Figure 3 which shows the same busbar system as on figure 2.

Longitudinal part 2 is called upstream part, since it is adjacent to the upstream side of the cell, upstream and downstream being defined with respect to the general direction of current flow. It first comprises a main busbar 20, which is straight and horizontal, and which extends along the whole length of part 2. This busbar 20 is rectangular in cross section, with vertical large sides. By way of example, its height H20 is between about 500 mm and about 1 , 100 mm, whereas its width W20 is between about 100 mm and about 300 mm. Busbar 20 is provided with a row of connectors (not shown on the figures), projecting downwards. In a known manner, each connector may be a flexible formed from stacked sheets and is intended to cooperate with the first end of a cathode block (not shown on the figures). Busbar 20 may be manufactured in one single piece or be assembled lengthwise from two half-rods, typically by welding; the welding seams are marked with reference number 121.

Longitudinal part 3 is called downstream part, since it is adjacent to the downstream side of the cell. It first comprises a main busbar 30, which is straight and horizontal, and which extends along the whole length of part 3. This busbar 30 is rectangular in cross section, with vertical large sides. By way of example, its height H30 is typically between about 300 mm and about 700 mm, whereas its width W30 is typically between about 100 mm and about 200 mm. Busbar 30 is provided with a row of connectors 31 , projecting downwards. In a known manner, each connector may be a flexible formed from stacked sheets and is intended to cooperate with the first end of a cathode block (not shown on the figures). Like busbar 20, busbar 30 may be manufactured in one single piece or be assembled lengthwise from two half-rods, typically by welding; the welding seams are marked with reference number 131. According to prior art downstream balancing circuits are connected to the downstream longitudinal part of the cathode busbar. They usually consist in a set of parallel busbar segments that are parallel to said downstream longitudinal cathode busbar. According to the invention, these downstream balancing circuits are no longer necessary, as will be explained in more detail below. Transversal part 4 is called duct end or duct part for a potline with current circulating clockwise; it is turned towards the line L2 of cells, facing the line L1 which includes present cell C2. Duct end and tap end would be interchanged for a potline with current circulating counter-clockwise. It may be formed in full thickness by one busbar, or may be formed by two parallel busbars, which may have equal or unequal cross-section, an inner busbar 41 and an outer busbar 42, which extend parallel the one to the other (the description will be given here for a transversal part 4 comprising two busbars 41 ,42). These parallel busbars are mutually distant, in order to define an intercalary space 43. Said intercalary space acts as an air gap that may provide some cooling of the busbars. Both busbars are rectangular in cross section, with vertical large sides. By way of example, each busbar has the same height H41 , which is between about 500 mm and about 1 , 100 mm, whereas each busbar has the same width W41 which is between about 200 mm and about 400 mm.

Transversal part 5 is called tap end or tap part in a potline with clockwise current, since it is turned opposite the other line L2 of cells. If the whole ring busbar is symmetrical in view of plane PY, which is actually the case in figures 2 and 3, the structure of this tap part is identical to that of duct part 4. On the drawings, the references of the components of part 5 are the same as those of part 4, apart from the fact that the first digit "5" replaces the first digit "4".

Each transversal part 4 or 5 is mechanically and electrically linked to a respective end of upstream longitudinal part 2 but electrically separated from downstream longitudinal part 3. To this end, an inner junction member 81 or 91 extends between inner rod 41 or 51 and facing parts of rod 20. Moreover, an outer junction member 82 or 92 extends between outer rod 42 or 52 and facing parts of main rod 20. Each junction member has an appropriate structure, so as to fulfil the above technical function. In the shown example, it is made of stacked sheets, the flexibility of which is sufficient to create a rounded shape.

Each transversal part 4, 5 is of uniform width. In an advantageous embodiment of the invention, the width and/or cross-section of the transversal duct-end part 4 is greater than that of the transversal tap-end part 5. For example, the duct end part 4 can have a width of about 465 mm, and the tap end part 5 can have a width of about 325 mm. This design creates asymmetric current and advantageously compensates the vertical magnetic field of the adjacent line.

As mentioned above, cathode current collected by the cathode busbar system of an upstream cell C2 is ultimately fed through anode risers into the anode beam 315 of the neighboring downstream cell C3. These anode risers 10a to 10d will be described in further detail. Anodes are arranged in two parallel rows, parallel to the longitudinal direction X-X of a pot. As a consequence, see in particular figure 2, the anode beam of a cell C3 has two sections, one section 316 close to the downstream longitudinal part 3 of the upstream cell C2, this section being called here "upstream anode beam", and one section 317 close to the upstream longitudinal part 2 of the downstream cell C4, this section being called here "downstream anode beam".

According to an essential feature of the invention, at least one of the anode risers 10 preferably two, and even more preferably all (as in the embodiments of figures 2,3,6 and 7) are split into two sections 11 +14,12+13 called, respectively, upstream anode riser 11 +14 (because it is connected to the upstream cathode busbars) and downstream anode riser 12+13 (because it is connected to downstream cathode busbars). The structure of one riser 10c will be described more in detail in reference to figure 4, bearing in mind that all risers 10a to 10d have substantially the same structure. Each riser section, i.e either upstream section or downstream section, comprises a rigid mast 11 or 12, as well as a flexible top 13 or 14. Each mast 11 or 12, which extends from cathode busbar, is typically manufactured as a single massive piece, made of aluminium. Each top

13 or 14 is typically manufactured from stacked aluminium sheet.

Mast section, formed by masts 11 and 12, is split according to at least one longitudinal plane that extends parallel to axis X-X. Bottom end of mast 11 or 12 is connected to facing part 30 or 35b of cathode bus bar, using any appropriate means.

Fixing means are provided between each mast 11 or 12, and each top 13 or 14. According to an advantageous feature of the invention, each fixing means comprises a respective fixing plate 15 or 16, which is for example welded to the top and bolted to the mast.

Free end of top 13 or 14 is connected to facing part of anode bus bar, using any appropriate means. In the embodiment of figure 2, tops 13 and 14 have substantially the same length, so that their free ends face substantially the same part of anode bus bar, i.e. the junction between branches 317 and 318, as shown on figure 2. However, tops 13 and

14 must be mutually isolated.

According to the present patent application, the term "connected" means a mechanical connection. Unless otherwise mentioned, this mechanical connection between two given members is also an electrical connection. These two members, which are mutually electrically connected, may of course be also electrically connected with other mechanically remote members, via intermediate connecting members. For each split riser 10, said upstream 11 and downstream 12 sections are electrically insulated from each other. This can be achieved by means of an appropriate electrical insulator 19 (shown on figures 4 and 5) which fills the intercalary space defined by facing so called split walls of masts 11 ,12. This insulator may be for example an air gap or an insulating plate. The thickness of this intercalary space, referenced as T19 on figure 4, is for example between 2 and 100 mm, in particular between 5 and 50 mm.

Each upstream anode riser 11 a, 11 b, 11c, 11 d is connected to the upstream longitudinal part 2, by means of the transversal parts 4, 5. As a consequence, the cathode current collected by the upstream longitudinal busbar 2 is fed into the upstream anode risers 11 a, 11 b, 11 c, 11 d. Each downstream anode riser 12a, 12b, 12c, 12d is connected to the downstream longitudinal busbar 3; as a consequence the cathode current collected by the downstream longitudinal busbar 3 is fed into the downstream anode risers 12a, 12b, 12c, 12d. These feeding routes are supported by an insulating gap 83, 93 between longitudinal downstream busbar 30 and, respectively, transversal tap end part 5 and transversal duct end part 4.

According to a first embodiment of the invention, the current collected by upstream anode risers 11 and downstream anode risers 12 is fed into the upstream anode beam, said upstream anode beam being in conductive contact with said downstream anode beam through a plurality of transversal anode beam sections 318a,318b,318c,318d. To achieve this, each upstream mast 11 (that may extend substantially vertically, as in the figures) is in conductive contact with flexible upstream top 13, and each downstream mast 12 (that may extend substantially vertically as in the figures) is in conductive contact with flexible downstream top 14.

Said flexible upstream and downstream tops 13, 14 are typically manufactured from stacked aluminium sheet; they are curved and separated by an appropriate electrical insulator 19, as explained above. The two split parts of the split risers are electrically insulated. Unlike the half bars of the transverse busbar sections 41 ,42 and 51 , 52 (as shown on figure 2), the two split parts 11 ,12 of the split risers 10 are not equipotential lines. In said first embodiment the equipotential point is the point where the split parts of the split risers meet in the upstream anode beam; this point is marked as Ea, Eb, Ec, Ed on figure 3.

In a variant of this embodiment of the invention shown on figures 2 and 3, the cathode busbar system according to the invention further comprises two conductive arms 6,7 that extend between longitudinal parts 2,3 underneath said shell and connect said longitudinal parts 2,3 together. In the embodiment shown on the figures 2 and 3 said arms are asymmetric with respect to said median longitudinal plane PX and symmetric or asymmetric with respect to said mean transversal plane PY.

Conductive arm 6 is called duct branch, since it is offset towards duct end 4, with respect to axis Y-Y'; it extends underneath the potshell. It comprises a main pole 61 , which extends parallel to Y-Y', under the surface of main plane PR, underneath the potshell. This pole is prolonged by two orthogonal branches 62 and 63, each of which extends under a respective longitudinal part 2 or 3 towards the head of the cell. The junctions between these branches 62, 63 and these parts 2,3 are different, depending on their downstream or upstream location.

Thus, upstream branch 62 is prolonged by an intermediate segment 64, which slopes both above and towards median axis Y-Y'. A terminal upright portion (not on the figures), made of stacked plates, links segment 64 and longitudinal upstream part 2. On the other hand, downstream branch 63 is directly linked to longitudinal part 3, via an upright portion 66, also made of stacked plates. In other words, the main difference between upstream and downstream zones of arm 6 is intermediate segment 64.

Branch 7 is called tap branch, since it is offset towards tap end 5, with respect to axis Y- Y'. Although, as explained above, duct branch 6 and tap branch 7 are symmetric with respect to axis Y-Y, the overall structure of this branch 7 is identical to that of branch 6. On the drawings, the references of the components of branch 7 are the same as those of branch 6, apart from the fact that the first digit "7" replaces the first digit "6".

Other geometries for conductive arms 6 and/or 7 can be used within the framework of the present invention. While Figure 2 shows a preferred variant of this aspect of the invention, in other variants said conductive arms are symmetric with respect to said median longitudinal plane PY and asymmetric with respect to said mean transversal plane PX, or they are asymmetric with respect to said median longitudinal plane PY and asymmetric with respect to said mean transversal plane PX.

Said conductive arms are connected to the closest upstream anode riser(s) by means of equilibrating branches 35a, 35b that extend parallel to the downstream longitudinal part 30 of the cathode busbar. As an example shown on figure 3, tap branch 7 collects current the current from the upstream longitudinal part 2 and carries it via upright portion 76 to said equilibrating branch 35b which is in conductive contact with at least one upstream anode riser 11 c, 11 d. According to prior art downstream balancing circuits are connected to the downstream longitudinal part of the cathode busbar. They usually consist in a set of parallel busbar segments that are parallel to said downstream longitudinal cathode busbar. According to the invention, these downstream balancing circuits are no longer necessary. A second embodiment of the invention is shown on figures 6 and 7. In this variant, the mechanical components that are identical to those of figure 2 are given the same reference numbers. However, the tops of the risers are different from those of figure 2, so that they are given the same reference numbers, added with 100. Tops 113 and 114 of this second embodiment have different lengths, so that their free ends face different locations of anode beam 315. Top 113 is connected to downstream anode beam 317, whereas top 114 is connected to upstream anode beam 316. Mechanical attachment is similar to that described for the above first embodiment. Therefore, the equipotential point at the anode beam of the downstream cell is shifted further downstream. More precisely, the upstream anode risers 11 +114 feed their current into the upstream anode beam 16, and the downstream anode risers 12+113 feed their current into the downstream anode beam 17. This further improves the electrical equilibrium of the busbar circuits, which can be seen form the fact that in this embodiment, the transversal anode beam sections 18 carry a very low electrical current.

This second embodiment can be combined with the variant with conductive arms 6,7, as explained above.

It should be noted that in the embodiment according to figures 1 to 3, the current is conducted clockwise, that is to say it enters the last cell Cm of line L1 upstream, crosses it downstream and then turns clockwise (in direction of the duct end) to line L2. Of course the invention applies also to counter-clockwise structures, and a person skilled in the art can easily adapt the cathode ring busbar system according as shown on the figures to counter-clockwise potlines.

The busbar system according to the invention can be manufactured from aluminium sections of appropriate cross section. In a known way, stacked aluminium sheets or plates and stacks of flexible aluminium sheets can be used for joining sections by welding and for flexible sections 13,14.

The invention has several advantages. The suppression of the zig-zag balancing circuit close to the downstream longitudinal section 3 allows to decrease the distance between two neighbouring pots C2,C3 when designing a new potline, by about 300 mm to 400 mm. Knowing that the cost of the building that houses the potline is a considerable contribution to the total cost of a new potline, saving 3 metres of length for each group of around twenty pots leads to a significant decrease in capital cost. The suppression of the zig-zag balancing circuit also saves metal in the busbar system, leading to a further decrease in capital cost.

The busbar system according to the invention however leads to a specific problem when the downstream pot is shut down for renovation. Indeed, if it is necessary to dismantle the anode risers, it is no longer possible to cut the pot out of the potline by using wedges only. Additionally, prior to electrically separating the anode risers 10 from the downstream anode beam 15, for each anode riser 10 the upstream mast 11 and the downstream mast 12 are short circuited by addition of a switch plate 98 that is set in conductive contact with both these upstream 11 and downstream 12 masts. This is shown in figure 5 for anode riser 10c: switch plate 98 is in conductive contact with upstream mast 11c and with downstream mast 12c. This allows disconnecting the cell C2 from its neighbouring downstream cell C3, for instance when the flexible tops 13c and 14c need to be dismantled for renovation of cell C3. Said switch plate 98 can be bolted on appropriate surfaces of the upstream and downstream masts 11 c, 12c. Figure 4 shows the same anode riser 10c in normal operation.

Claims

1. An anode riser (10) for an electrolytic cell, in particular for a cell suitable for the Hall-Heroult electrolysis process, said riser being intended to electrically connect a cathode busbar (2-5) of an upstream cell and an anode busbar (315) of a downstream cell,

said anode riser comprising a first section (11 , 14) and a second section (12, 13), each section being provided with respective first and second connection means on cathode busbar and with respective first and second connection means on anode busbar, said anode riser further comprising means (19) for electric insulation between said first section and said second section.

2. An anode riser according to claim 1 , wherein means for electric insulation comprise an insulating material (19) filling at least part of an intercalary space defined by facing so called split walls of first and second sections.

3. An anode riser according to claim 1 or 2, wherein said intercalary space has a substantially constant thickness (T19) over at least part of the length of said riser, at least in the vicinity of first connection means.

4. An anode riser according to claim 3, wherein said thickness (T19) is comprised between 2 mm and 20 mm.

5. An anode riser according to any of claims 1 to 4, wherein said riser comprises a rigid mast (1 1 , 12), intended to be connected on cathode busbar, as well as a flexible top

(13, 14), extending from said mast and being intended to be connected on anode busbar, said mast comprising a first mast section (11) and a second mast section (12), said top comprising a first top section (13) and a second top section (14).

6. An anode riser according to claim 5, wherein said riser comprises first fixing means (15) between first mast section and first top section, as well as second fixing means (16) between second mast section and second top section.

7. An anode riser according to claim 6, wherein each fixing means (15, 16) is a removable fixing means.

8. An anode riser according to claim 7, wherein each removable fixing means comprise a fixing plate (15,16), said plate being permanently fixed, in particular by welding, to one amongst mast section and top section, and being removably fixed, in particular by bolting, to the other amongst mast section and top section.

9. An anode riser according to any of claims 5 to 8, wherein said first top section (13) and second top section (14) have substantially the same length.

10. An anode riser according to any of claims 5 to 9, wherein one amongst said first top section (113) and second top section (114) is substantially longer than the other, so that said first top section and second top section may directly contact different portions of anode bus bar.

1 1. A busbar arrangement for two adjacent upstream and downstream electrolytic cells of substantially rectangular shape, in particular for cells suitable for the Hall-Heroult electrolysis process,

a lateral lining defining together with the cathode a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process, said cathode and lateral lining being contained in an outer metallic shell, and each electrolytic cell further comprising a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one anode and at least one metallic anode rod connected to an anode busbar (so- called anode beam),

said arrangement comprising a cathode busbar system (2-5) as well as an anode busbar system (315)

said cathode busbar system (2-5) comprising two opposite longitudinal parts (2,3) each intended to extend along the long sides of the upstream cell, and two opposite transversal parts (4,5) intended to extend along the ends of the upstream cell,

said anode busbar system (315) comprising two opposite longitudinal parts (316,317) and at least two opposite transversal parts (318a, 318d) extending between said longitudinal parts (316,317)

said arrangement further comprising at least one anode riser (10a-10d) according to any of claims 1 to 10.

12. A busbar arrangement according to claim 11 , wherein one (3) of said longitudinal parts of said cathode busbar system comprises first and second electrically isolated sections (30,35b), the first section of said riser being connected to first section of said longitudinal part, whereas second section of said riser is connected to second section of said longitudinal part.

13. A busbar arrangement according to claim 11 or 12, wherein the first section (1 1 , 14) and the second section (12, 13) of said riser are connected to a first longitudinal part (316) of the anode bus bar of a neighbouring downstream cell.

14. A busbar arrangement according to claim 1 1 or 12, wherein the first section (11 , 14) of said riser is connected to a first longitudinal part (316) of anode bus bar, whereas second section of said riser is connected to a second longitudinal part (317) of anode bus bar of a neighbouring downstream cell.

15. Electrolytic cell of substantially rectangular shape suitable for the Hall-Heroult electrolysis process, comprising

said electrolytic cell being characterized in that it comprises a riser according to any of claims 1 to 10.

16. An aluminium electrolysis plant comprising at least one line (L1 , L2) of electrolysis cells (C1 , Cn) of substantially rectangular shape, said cells being arranged side by side,

each cell comprising

a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks, each cathode block being provided with at least one current collector bar and two electrical connections points, a lateral lining defining together with the cathode a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process, said cathode and lateral lining being and lining being contained in an outer metallic shell,

said plant further comprising means for electrically connecting said cells in series and for connecting the cathode busbar of a cell to the anode beam of a downstream cell, characterized in that more than 80 % of the electrolysis cells in at least one line (L1 , L2), and preferably each electrolysis cell in said line, is an electrolysis cell according to claim 15.

17. An aluminium electrolysis plant comprising at least one line (L1 , L2) of electrolysis cells (C1 , Cn) of substantially rectangular shape, said cells being arranged side by side,

each cell comprising

said plant further comprising means for electrically connecting said cells in series and for connecting the cathode busbar of a cell to the anode beam of a downstream cell, characterized in that more than 80 % of the couple of adjacent electrolysis cells in at least one line (L1 , L2), and preferably each couple of electrolysis cells in said line are electrically connected by a busbar arrangement according to any of claims 11 to 14.

18. Process for making aluminium by the Hall-Heroult electrolysis process using electrolytic cells of substantially rectangular shape, characterized in that said process is carried out in an aluminium electrolysis plant according to claim 16 or 17.